Electron microscopy provides significantly higher resolution and greater depth of focus than optical microscopy. In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface.
In a transmission electron microscope (TEM), a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
There are several methods for preparing a thin sample for viewing with a TEM or STEM. Some methods entail extracting a sample without destroying the entire substrate from which the sample is extracted. Other methods require destroying the substrate to extract the sample. One method, described by Anderson et al. in “Combined Tripod Polishing and FIB Method for Preparing Semiconductor Plan View Specimens,” Materials Research Society Proceedings, Vol. 480, pp. 187-192 (1997), cuts a thin strip for the substrate using a diamond saw, mechanically polishes the sample to a specified thickness, and then further thins the sample using a focused ion beam. Another method, described in E. C. G. Kirk et al., “Cross-Sectional Transmission Electron Microscopy of Precisely Selected Regions from Semiconductor Devices,” Inst. Phys. Conf. Ser. No. 100, Section 7, (1989) entails cutting a portion from a substrate using a diamond saw and then using a focused ion beam to produce a thin sample on a part of a substrate portion.
U.S. Pat. No. 6,841,788 to Robinson describes using a femtosecond laser to cut through a semiconductor wafer to free a plug or block as thick as the wafer, that is, about 750 μm thick. A thin sample suitable for TEM or STEM viewing is formed in the top of the block. Laser drilled guide holes are cut in the block and used to pick it up. Robinson teaches that by removing the block, one avoids the removal a “fragile member,” that is, the thin sample viewable in a TEM. Although the method of Robinson does not destroy the entire wafer, the hole in the wafer after the plug is removed renders it unsuitable for further processing, because the hole will harbor contaminants.
One method that allows a sample to be extracted without destroying the substrate described in U.S. Pat. No. 5,270,552 to Ohnishi et al., which describes using a focused ion beam to free a sample from a substrate and to weld a probe to the sample using ion beam deposition to transport the sample. Herlinger et al., “TEM Sample Preparation Using a Focused Ion Beam and a Probe Manipulator,” Proceedings of the 22nd International Symposium for Testing and Failure Analysis, p. 199-205 (1996) describes using a focused ion beam to free a sample from a substrate, removing the sample from the vacuum chamber, and then moving the sample to a TEM sample holder using a probe to which the sample adheres by electrostatic attraction. Another method include the use of tweezer-like gripper to grasp the sample. All these methods are slow and time consuming.
In some extraction methods, such as that described by Ohnishi et al., the extracted sample is a “chunk” that needs to be thinned extensively before it can be viewed on a TEM or STEM. In other embodiment, such as that described in Herlinger et al., the extracted sample is a thin lamella, that requires only minor finishing before TEM viewing.